All-mass MS/MS method and apparatus

ABSTRACT

A method of obtaining a mass spectrum of elements in a sample is disclosed. Sample precursor ions having a mass to charge ratio M/Z are generated, and fragmented at a dissociation site, so as to produce fragment ions of mass to charge ratio m/z. The fragment ions are guided into an ion trap of the electrostatic or “Orbitrap” type, the fragment ions entering the trap in groups dependent upon the precursor ions M/Z. The mass to charge ratio of each group is determined from the axial movement of ions in the trap. The electric field in the trap is distorted. Ions of the same m/z, that are derived from different pre-cursor ions, are then separated, because the electric field distortion causes the axial movement to become dependent upon factors other than m/z alone.

FIELD OF THE INVENTION

This invention relates to a method and apparatus of mass spectrometry,and in particular all-mass MS/MS using Fourier Transform electrostaticion traps.

BACKGROUND OF THE INVENTION

Tandem mass spectrometry, or MS/MS, is a well known technique used toimprove a spectrometer's signal-to-noise ratio and which can provide theability to unambiguously identify analyte ions. Whilst the signalintensity may be reduced in MS/MS (when compared with single stage MStechniques), the reduction in noise level is much greater.

Tandem mass spectrometers have been used to analyse a wide range ofmaterials, including organic substances such as pharmaceuticalcompounds, environment compounds and biomolecules. They are particularlyuseful, for example, for DNA and protein sequencing. In suchapplications there is an ever increasing desire for improving theanalysis time. At present, liquid chromatography separation methods canbe used to obtain mass spectra of samples. LC techniques often requirethe use of “peak-parking” to obtain full spectral information and thereis a general consensus among persons skilled in the art that theacquisition time needed to obtain complete information about all peaksin a mass spectrum adds a significant time burden to research programs.Thus, there is a desire to move to higher throughput MS/MS.

Structure elucidation of ionised molecules can be carried out usingtandem mass spectrometry, where a precursor ion is selected at a firststage of analysis or in a first mass analyser (MS1). This precursor ionis subjected to fragmentation, typically in a collision cell, andfragment ions are analysed in a second stage analyser (MS2). This widelyused fragmentation method is known as collision induced dissociation(CID). However, other suitable dissociation methods include surfaceinduced dissociation (SID), photo-induced dissociation (PID) ormetastable decay.

Presently, there are several types of tandem mass spectrometergeometries known in the art in various geometric arrangements, includingsequential in space, sequential in time, and sequential in time andspace.

Known sequential in space geometries include magnetic sector hybrids, ofwhich some known systems are disclosed in Tandem Mass Spectrometryedited by W F McLafferty and published by Wiley Inter-Science, New York,1983; quadrupole time-of-flight (TOF) spectrometer described by Mauriceet al in Rapid Communications in Mass Spectrometry, 10 (1996) 889-896;or TOF-TOF described in U.S. Pat. No. 5,464,985. As described inHoagland-Hyzer's paper, Analytical Chemistry 72 (2000) 2734-2740, thefirst TOF analyser could be replaced by a separation device based on adifferent principle of ion mobility. The relatively slow time-scale ofprecursor ion separation in an ion mobility spectrometer allows theacquisition of a number of TOF spectra over each scan. If fragmentationmeans are provided between the ion mobility spectrometer and the TOFdetector, then all-mass MS/MS becomes possible, albeit with very lowprecursor ion resolution.

Sequential in time mass spectrometers include ion traps, such as thePaul trap described by March et al in Quadrupole Storage MassSpectrometry published by John Wiley, Chichester, 1989; or FTICRspectrometers as described by A G Marshall et al, Optical and MassSpectrometry, Elsevier, Amsterdam 1990; or LT Spectrometers such as theone disclosed in U.S. Pat. No. 5,420,425.

Known sequential in time and space spectrometers include 3D trap-TOF(such as the one disclosed in WO 99/39368 where the TOF is used only forhigh mass accuracy and acquisition of all the fragments at once); FT-ICRsuch as the spectrometer disclosed by Belov et al in AnalyticalChemistry, volume 73, number 2, Jan. 15, 2001, page 253 (which islimited by the slow acquisition time of the MS2); or LT-TOFspectrometers, (for example as disclosed in U.S. Pat. No. 6,011,259,which transmits only one precursor ion but which the inventors claim tohave achieved a 100% duty cycle).

All of these existing mass spectrometers are only able to providesequential analysis of MS/MS spectra, that is, one precursor mass at atime. Put another way, it is not possible to provide an all-mass spectrafor all precursor masses in a single analysis using these existing massspectrometers. Insufficient dynamic range and acquisition speed of MS-2mass spectrometers are considered to be a limiting factor in thespectrometer's ability.

This dynamic range and acquisition speed problem has been partiallyaddressed for Fourier Transform ion cyclotron resonance (FTICR) massspectrometers, as described in Analytical Chemistry, 1990, 62, 698-703(Williams E R et al) and in the Journal of the American ChemicalSociety, 115 (1993) 7854, Ross C W et al. Two different multiplexapproaches have been demonstrated which take advantage of amulti-channel arrangement. These are as follows:

Two Dimensional Hadamard/FTICR Mass Spectrometry

In this method, a sequence of linearly independent combinations ofprecursor ions are selected for fragmentation to yield a combination offragment mass spectra. Encoding/decoding of the acquired “masked”spectra is provided by Hadamard transform algorithms. Williams E R et al(referred to above) have shown that for N different precursor ions, agiven signal to noise ratio could be achieved in experiments having areduced spectra acquisition time of N/4-fold.

Two Dimensional Fourier/FTICR Mass Spectrometry

This method uses an excitation waveform to excite all the precursorions. This provides different excitation states for different masses ofprecursor ions. Using stored waveform inverse Fourier Transform (SWIFT)methods, the excitation waveform is a sinusoidal function of precursorion frequency, with the frequency of the sinusoidal function increasingfrom one acquisition to another. As a result, the intensities offragment ions for a particular precursor ion are also modulatedaccording to the applied excitation. Inverse 2D Fourier Transformapplied to a set of transients results in a 2D map which unequivocallyrelates fragment ions to their precursors.

According to Marshall A G (referred to above) the first method requiressubstantially less data storage and the second method requires no priorknowledge of the precursor ion spectrum. However, in practical terms,both methods are not compatible with commonly used separationtechniques, for instance HPLC or CE. This is due to the relatively lowspeed of FTICR acquisition (which is presently no faster than a fewspectra per second), and a relatively large number of spectra required.Also, unless the LC separation method is artificially “paused” usingrelatively cumbersome “peak parking” methods, the analyte can exhibitsignificant intensity changes within a few seconds (in the most widelyused separation methods). Further, the use of peak parking methods cangreatly increase the time to acquire spectra.

GB-A-2,378,312 and WO-A-02/078046 describes a mass spectrometer methodand apparatus using an electrostatic trap. A brief description isprovided of some MS/MS modes available for this arrangement. However, itdoes not address any problems associated with all-mass MS/MS analysis inthe trap. The precursor ions are ejected from a storage quadrupole, andfocussed into a coherent packet by TOF focussing so that the ions havingthe same m/z enter the electrostatic trap at substantially the samemoment in time.

The trajectories of ions in an electrostatic trap are described byMakarov in “Electrostatic Axially Harmonic Orbital Trapping: A HighPerformance Technique of Mass Analysis”, Journal of AnalyticalChemistry, v. 72, p 1156-1162 (2000). From the equations of motionpresented in Makarov's paper, it follows that the axial frequency isindependent of the energy and the position of ions in the trap (or phaseof ions as they enter to trap). Thus, the axial frequency of ion motionis used for mass analysis.

SUMMARY OF THE INVENTION

The present invention provides a method of mass spectrometry using anion trap, the method comprising: a) generating a plurality of precursorions from a sample, each ion having a mass to charge ratio selected froma first finite range of mass to charge ratios M₁/Z₁, M₂/Z₂, M₂/Z₃ . . .M_(N)/Z_(N); b) causing at least some of the plurality of precursor ionsto dissociate, so as to generate a plurality of fragment ions, each ofwhich has a mass to charge ratio selected from a second finite range ofmass to charge ratios m₁/z₁, m₂/z₂, m₃/z₃ . . . m_(n)/z_(n); c)directing the fragment ions into an ion trap, the ion trap includingmeans for generating an electromagnetic field which allows trapping ofions in at least one direction thereof, the ions entering the trap ingroups at a time which depends upon the mass to charge ratio of theprecursor ions; d) determining the mass to charge ratio of ions in atleast one of the groups of ions, based upon a parameter of motion of theions in that or those groups in the said electromagnetic field in thetrap; and e) distorting the electromagnetic field in the trap so as topermit separate detection of fragment ions within the trap which havethe same mass to charge ratio, but which are derived from differentprecursor ions.

Preferably, the trap is an electrostatic trap. Advantageously, themethod can distinguish two or more fragmented ion groups having the samemass to charge ratio m/z, each being derived from different precursorion groups with different M₁/Z₁, M₂/Z₂ etc, from one another when theelectric field is distorted. The distortion causes the frequency of(axial) oscillation of one ion group to change relative to the other iongroup. Thus, where the two ion groups were previous undistinguishablefrom one another, their change of axial frequency relative to each othernow renders them distinguishable. The location might be either thelocation of ion formation (for instance, if MALDI ion sources are used),or the location at which ions are released from intermediate storage inan RF trapping device, for example.

It is possible to “label” each ion group derived from differentprecursor ions because any one of the parameters (e.g. amplitude ofmovement of each group in the electrostatic trap, or ion energy in eachgroup, or the initial phase of oscillation of each group in theelectrostatic trap) is dependent on T, in the electrostatic trap (whereT is the TOF of an ion from its place of release to the electrostatictrap entrance), and T is in turn dependent on the mass to charge ratioof the precursor and/or fragment ions.

The method has further advantages of being able to acquire a fullspectrum for each of the many precursor ions in one individual spectrum,if for example, detection is performed in the electrostatic field usingimage current detection methods.

Determination of the differences of movement amplitude and energies foreach of the fragmented ion groups can be achieved by distorting theelectric field in the electrostatic trap. In this way, the axialfrequency of trajectories for each of the fragment ions (having the samemass to charge ratio m₁/z₁) in the trap is no longer independent of ionparameters.

Preferably the electric field is distorted locally by applying a voltageto an electrode. The electric field distortion can be arranged such thatthe axial oscillation frequency of a fragmented ion relatively close tothe distortion is different to the axial oscillation frequency of theother fragmented ion, relatively distant from the distortion. Thus,fragment ions with the same mass to charge ratio m₁/z₁, but beingderived from precursor ions with different mass to charge ratios M₁/Z₁and M₂/Z₂ can be distinguished from one another. A method for all-massMS/MS is therefore achieved.

Embodiments of the present invention are capable of improving the speedof analysis by five to ten times, at least, compared to LC peak parkingtechniques.

The present invention also provides a mass spectrometer comprising: anion source, arranged to supply a plurality of sample ions to beanalysed; means for directing the sample ions towards a dissociationlocation, the sample ions arriving at the said dissociation location asa plurality of groups of precursor ions in accordance with their mass tocharge ratios selected from the range M₁/Z₁, M₂/Z₂, M₃/Z₃ . . .M_(N)/Z_(N); an ion trap having a trap entrance, the ion trap beingarranged to receive groups of fragment ions generated by dissociation ofthe precursor ions at the dissociation location, each group of fragmentions having a mass to charge ratio selected from the range m₁/z₁, m₂/z₂,m₃/z₃ . . . m_(n)/z_(n), the ion trap further comprising trap electrodesconfigured to generate a trapping field within the ion trap, so thatunfragmented precursor ions and/or fragment ions entering the trap aretrapped in at least one axial direction thereof by the said trappingfield and have a parameter of movement related solely to the mass tocharge ratio of the ion; detection means to permit determination of themass to charge ratio of an ion group based upon the said parameter ofmovement; and at least on electric field distorting electrode arrangedto provide a distortion of the trapping field so as to permit thedetection means to detect separate groups of fragment ions in the iontrap which have the same mass to charge ratio, m₁/z₁, but which havederived from precursor ions having at least two different mass to chargeratios M₁/Z₁, M₂/Z₂.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example, and withreference to the following drawings, in which;

FIG. 1 is a schematic diagram of an apparatus used by the presentinvention;

FIG. 2 is a schematic diagram showing details of the electrostatic trapshown in FIG. 1;

FIG. 3 is a schematic diagram showing the orbital paths of two ionshaving the same m/z, but different energy;

FIG. 4 is a schematic diagram showing the variation of voltage appliedto an electrode over time;

FIG. 5 is a schematic diagram showing the envelope of a detectedtransient ion in the orbitrap;

FIG. 6 is a schematic diagram of a mass spectrum acquired before T_(D)using embodiments of the present invention;

FIG. 7 is a schematic diagram showing a mass spectrum relating to thespectrum of FIG. 6, except that the phase of each peak detected isshown;

FIG. 8 is a mass spectrum acquired after T_(D) using an embodiment ofthe present invention;

FIG. 9 is a schematic diagram showing the mass spectrum of FIG. 8,except that the phase of each peak detected is shown; and

FIG. 10 to 13 each show various alternative arrangements of anelectrostatic trap embodying the present invention.

We have realised that Fourier Transform mass spectrometers have thepotential for acquiring an MS/MS spectrum from multiple precursor ionsin a single scan, which can greatly reduce the time burden on acquiringa spectrum to a level at least comparable with, or better than LC.

The present invention is described with reference to an electrostatictrap according to the trap disclosed in GB-A-2,378,312, WO-A-96/30930and Makarov's paper (referred to previously) and these documents arehereby incorporated by reference. Reference is made to this trapthroughout the description as an “orbitrap”. Of course, otherarrangements of electrostatic traps can be used and this invention isnot limited to use with the specific embodiment disclosed herein and inthese references. Other electrostatic traps might include arrangementsof multi-reflecting mirrors of planar, circular, elliptical, or othercross-section. In other words, the present invention could be applied toany electrode structure sustained at high vacuum which provides multiplereflections and isochronous ion motion in at least one direction. It isnot necessary to describe the orbitrap in great detail in this documentand reference is made to the documents cited above in this paragraph.The present invention may also, in principle, be applied to atraditional FTICR, although this would require development ofsophisticated ion injection and excitation techniques. For example, someelectrodes of the FTICR cell, particularly the detection electrodes,could be energised to provide controlled field perturbation.

Preferably, for accurate detection to take place, the orbitrap requiresions to be injected into the trap with sufficient coherence to preventsmearing of the ion signal. Thus, it is necessary to ensure that groupsof ions of a given mass to charge ratio arrive as a tightly focussedbunch at, or adjacent to, the electrostatic trap entrance. Such bunchesor packets are ideally suited for electrostatic traps, because the fullwidth half maximum (FWHM) of each of the ion packet's TOF distribution(for a given mass to charge ratio) is less than the period ofoscillation of sample ions having that mass to charge ratio when in theelectrostatic trap. Reference is made to U.S. Pat. No. 5,886,346 andGB-A-2,378,312 which describes particular restrictions on the releasepotential and these two documents are hereby incorporated by reference.Alternatively, a pulsed ion source (for example using short laserpulses) can be employed with similar effect.

Referring to FIG. 1, a mass spectrometer 10 is shown. The massspectrometer comprises a continuous or pulsed ion source 12, such as anelectron impact source, an electrospray source (with or without aCollision RF multipole), a matrix assisted laser desorption andionization (MALDI) source, again with or without a Collision RFmultipole, and so forth. In FIG. 1 an electrospray ion source 12 isshown.

Nebulised ions from the ion source 12 enter an ion source block 16having an entrance cone 14 and an exit cone 18. As is described inWO-A-98/49710, the exit cone 18 has an entrance at 90° to the ion flowin the block 16 so that it acts as a skimmer to prevent streaming ofions into the subsequent mass analysis components.

A first component downstream of the exit cone 18 is a collisionalmultipole (or ion cooler) 20 which reduces the energy of the sample ionsfrom the ion source 12. Cooled ions exit the collisional multipole 20through an aperture 22 and arrive at a quadrupole mass filter 24 whichis supplied with a DC voltage upon which is superimposed an arbitrary RFsignal. This mass filter extracts only those ions within a window ofmass to charge ratios of interest, and the chosen ions are then releasedinto linear trap 30. The ion trap 30 is segmented, in the embodimentshown in FIG. 1, into an entrance segment 40 and an exit segment 50.Though only two segments are shown in FIG. 1 it is understood that threeor more segments could be employed.

As is familiar to those skilled in the art, the linear trap 30 may alsocontain facilities for resonance or mass selective instability scans, toprovide data dependant excitation, fragmentation or elimination ofselected mass to charge ratios.

Ions are ejected from the trap 30. In accordance with a convention nowdefined, these ions, which are (as will be understood from thefollowing) precursor ions, have one of a range of mass to charge ratiosM_(A)/Z_(A), M_(B)/Z_(B), M_(C)/Z_(C) . . . M_(N)/Z_(N), where M_(N) ismass and Z_(N) is charge of an N^(th) one of the range of M/Z ratios ofthe precursor ions.

Downstream of the exit electrode is a deflection lens arrangement 90including deflectors 100, 110. The deflection lens arrangement isarranged to deflect the ions exiting trap 30 in such a way that there isno direct line of sight connecting the interior of the linear trap 30with the interior of an electrostatic orbitrap 130, downstream of thedeflection lens arrangement 90. Thus, streaming of gas molecules fromthe relatively high pressure linear trap into the relatively lowpressure orbitrap 130 is prevented. The deflection lens arrangement 90also acts as a differential pumping aperture. Downstream of thedeflection lens arrangement is a conductivity restrictor 120. Thissustains a pressure differential between the orbitrap 130 and the lensarrangement 90.

Ions exiting the deflection lens through the conductivity restrictorarrive at an SID surface 192, on the optical axis of the ion beam fromthe transfer lens arrangement 90. Here, the ions collide with thesurface 192 and dissociate into fragment ions having a mass to chargeratio which will be in general different to that of the precursor ion.In keeping with the convention defined above for the precursor ions, themass to charge ratio of the resultant fragment ions is one ofm_(a)/z_(a), m_(b)/z_(b), m_(c)/z_(c) . . . m_(n)/z_(n), where m_(n) andz_(n) are the mass and charge of an n^(th) one of the range of m/zratios of the fragment ions.

The fragment ions, and any remaining precursor ions are reflected fromthe surface and arrive at the orbitrap entrance. The orbitrap 130 has acentral electrode 140 (as may be better seen with reference now to FIG.2). The central electrode is connected to a high voltage amplifier 150.

The orbitrap also preferably contains an outer electrode split into twoouter electrode parts 160, 170. Each of the two outer electrode parts isconnected to a differential amplifier 180. Preferably this differentialamplifier is maintained at virtual ground.

Referring once more to FIG. 1, downstream of the orbitrap is a secondaryelectron multiplier 190 located to the side of the orbitrap 130. Alsoshown in FIG. 1 is an SID surface voltage supply 194. In an alternativeembodiment, a deceleration gap can be provided between a grid (placed infront of the CID surface) and the surface. Ions pass through the gridinto the gap, where they experience a deceleration force caused by anoffset voltage applied to the grid. In this way, the collision energybetween the ions and the surface can be reduced in a controlled manner.

The system, and in particular the voltages supplied to the various partsof the system, is. controlled by a data acquisition system which doesnot form part of the present invention. Likewise, a vacuum envelope isalso provided to allow differential pumping of the system. Again this isnot shown in the figures although the typical pressures are indicated inFIG. 1.

The operation of the system, from ions leaving the ion source 12,entering the segmented linear trap 30, being released from the trap anddeflected by the lens arrangement 90 are described in GB 0126764.0. Theoperation of the system up to release of the ions from a linear trapdoes not form part of the present invention. Accordingly no furtherdetailed discussion of this aspect of the apparatus is necessary in thisdocument.

The embodiment shown in FIG. 1 has the SID surface placed behind thetrap, in a reflective geometry, so that ions pass through the orbitrapwithout being deflected into the trap entrance (there being no voltageapplied to the deflection electrode 200 or electrode 140 at this stage).The ions interact with the collision surface 192, dissociating intofragment ions and are reflected back from the surface into the orbitrap.At this stage, a voltage is applied to the electrode 200 and the ionsare deflected into the orbitrap.

The energy of the collisions with the surface (and also the energyspread on the resulting fragments) can be regulated by a retardingvoltage 194 applied to the SID surface. The distance between the SIDsurface and the trap 130 is chosen with ion optical considerations inmind, as well as the required mass range. In the preferred embodimentthe ions leave the ion trap 30 and are time of flight (TOF) focused ontothe SID surface. As a result, the ions arrive at the SID surface indiscrete bunches according to the mass to charge ratio; each bunch hasions of mass to charge ratio M_(A)/Z_(A), M_(B)/Z_(B), . . .M_(N)/Z_(N), as defined above. There is no TOF focussing of theprecursor or fragment ions from the SID surface into the orbitrap'sentrance. The SID is located as close to the orbitrap's entrance as ispractical so that any spreading or smearing of ions is minimised. Thedistance L between the SID site and the entrance is preferably between50-100 mm. As a result, the additional broadening of an ion packet, dL,from the SID surface to the orbitrap's entrance is negligible, andtypically less than 0.5 to 1 mm (as the energy distribution of fragmentions leaving the SID is 10-20 eV and the acceleration voltage is of theorder of 1 keV). It is to be understood, of course, that thisarrangement is merely a preferred embodiment and other forms ofdissociation known in the art may also be used. The principles ofreducing smearing by maintaining a short distance between thedissociation site and the orbitrap's entrance remain the same, whateverthe form of dissociation.

The skilled artisan will appreciate that photo-induced dissociation(PID), using an impulse laser, may be employed. PID utilises therelatively high peak power of a pulsed laser to dissociate the precursorions. The dissociation is preferably made in a region where theprecursor ions have a lower kinetic energy so that the fragment ionshave energies within the energy acceptance of the trap. Furthermore,collision induced dissociation (CID) can be carried out in a region oflower kinetic energy of precursor ions, preferably in a relativelyshort, high pressure collision cell. The cell should be arranged toavoid significant broadening of all the time-of-flight distributionsfrom the linear trap 30. Thus, the time-of-flight of ions inside the CIDcell is desirably less than, and more preferably, very much less, thanboth the TOF of ions from the linear trap to the cell, and from the cellto the orbitrap's entrance. At present, we believe that fragmentation byCID is the least preferable approach because of the inherently stricthigh vacuum limitations of electrostatic traps.

In the operation of the preferred embodiment, a pulse of precursor (or“parent”) ions is released from the linear ion trap 30. The ionsseparate into discrete groups according to their times-of-flight duringtheir transition from the storage quadrupole or sample plate to thedissociation site, the TOF separation in turn being related to thevalue, n, in the mass to charge ratio M_(N)/Z_(N) as defined previously.

Each group, or packet of ions (which now comprises ions of substantiallythe same mass to charge ratio M/Z) collides with the dissociation site.Here, some precursor ions are fragmented into fragment ions with lowerenergy (in the order of several eV) than the precursor ions' energy.Fragmentation using SID is essentially an instantaneous process. Thus,the fragment ions are ejected from the dissociation site in groups orpackets. These fragmented ion groups have differing TOFs from thedissociation site to the orbitrap entrance, according to theirmass-to-charge ratios m_(n)/z_(n). Each bunch of precursor ions ofM_(N)/Z_(N) may produce fragment ions of various mass to charge ratiosm_(a)/z_(a), m_(b)/z_(b), . . . m_(n)/z_(n). Some unfragmented ions ofmass to charge ratio M_(A)/Z_(A), M_(B)/Z_(B), M_(C)/Z_(C) . . .M_(N)/Z_(N) may also remain. Hence, fragment ions and any remainingprecursor ions are injected off axis into the increasing electric fieldof the orbitrap as coherent groups, depending on their mass-to-charge.Coherent packs of the precursor and fragment ions are thus formed in theorbitrap, with each pack having ions of the same mass to charge ratiom_(a)/z_(a), m_(b)/z_(b), m_(c)/z_(c) . . . m_(n)/z_(n); M_(A)/Z_(A),M_(B)/Z_(B), M_(C)/Z_(C) . . . M_(N)/Z_(N).

During ion injection a voltage 150, applied to the central electrode 140of the orbitrap, is ramped. As explained in Makarov's paper (referencedabove), this ramping voltage is utilised to “squeeze” ions closer to thecentral electrode and can increase the mass range of trapped ions. Thetime constant of this electric field increase is typically 20 to 100microseconds, but depends on the mass range of the ions to be trapped.

During normal operation, the (ideal) electric field in the orbitrap ishyper-logarithmic, due to the shape of the central and outer electrodes.Such a field creates a potential well along the longitudinal axisdirection which causes ion trapping in that potential well provided thatthe ion incident energy is not too great for the ion to escape. As thevoltage applied to the centre of electrode 140 increases, the electricfield intensity increases and therefore the force acting on the ionstowards the longitudinal axis increases, thus decreasing the radius ofspiral of the ions. As a result, the ions are forced to rotate inspirals of smaller radius as the sides of the potential well increase ingradient.

As discussed in the prior art, there are three characteristicfrequencies of oscillation within the hyper-logarithmic field. The firstis the harmonic motion of the ions in the axial direction where the ionsoscillate in the potential well with a frequency independent of ionenergy. The second characteristic frequency is oscillation in the radialdirection since not all of the trajectories are circular. The thirdfrequency characteristic of the trapped ions is the frequency of angularrotation. The moment T of an ion pack entering the orbitrap electricfield is a function of the mass to charge ratio of the ions in it (i.e.,in general, m_(n)/zn or M_(N)/Z_(N)) and is defined in equation 1provided below:

$\begin{matrix}{{T\left( {{m_{n}/z_{n}},{M_{N}/Z_{N}}} \right)} \approx {t_{o} + {{TOF}_{1}\left( {M_{N}/Z_{N}} \right)} + {{{TOF}_{2}\left( {M_{N}/Z_{N}} \right)} \cdot \sqrt{\frac{m_{n}/z_{n}}{M_{N}/Z_{N}}}}}} & (1)\end{matrix}$where t_(o) is the moment of ion formation or release from the trap;TOF₁(M_(N)/Z_(N)) is the time-of-flight of precursor ions of mass tocharge ratio M_(N)/Z_(N) from the place of ion release or ion formationto the collision surface; TOF₂(M_(N)/Z_(N)) is the time-of-flight ofprecursor ions of mass to charge ratio M_(N)/Z_(N) (i.e. the same massto charge ratio as the ions incident upon the collision surface butwhich have failed to dissociate), from the collision surface to theentrance to the orbitrap; and m_(n)/z_(n) is the mass to charge ratio offragment ions produced upon collision, from the precursor ions of massto charge ratio M_(N)/Z_(N). It will also be understood that equation 1links precursor ions of one specific mass to charge ratio M_(N)/Z_(N) toa single packet of fragment ions each having a mass to charge ratiom_(n)/z_(n), although a similar equation may be applied to estimate themoment T′ for fragment ions of mass to charge ratio m_(a)/z_(a), forexample, also deriving from the same precursor packet having M_(N)/Z_(N)simply by substituting m_(a)/z_(a) for m_(n)/z_(n) in equation 1. Ionscould also be generated from a solid or liquid surface using MALDI, fastatom bombardment (FAB), secondary ion bombardment (SIMS) or any otherpulsed ionization method. In these cases, t₀ is the moment of ionformation. The effects of energy release, energy spread and otherconstants or variables are not included in equation 1 for clarityreasons.

There are parameters which are dependent on ion mass-to-charge ratio dueto the separation of the ions into groups according to their TOF fromthe quadrupole trap. These parameters include the amplitude of movementduring detection in the orbitrap (for example, radial or axialamplitudes), the ion energy during detection, and the initial phase ofion oscillations (which is dependent on T). Any of these parameters canbe used to “label” the precursor or fragment ions.

It is preferable that the fragment ions are formed on a timescale suchthat TOF effects do not disrupt the fragmented ion package coherence toan extent which might affect detection (eg. because of smearing causedby energy spread). The parameters of the fragment ions may differ fromthose of the precursor ions. However, the fragment ions can beunequivocally related to their precursor ion's parameters. This isachieved in the following manner.

In a preferred embodiment, detection of the ion's axial oscillationfrequencies in the trap starts at a predetermined detection time T_(det)after t₀. T_(det) is typically several tens of milliseconds (forinstance 60 ms or more) after t₀ and the TOF of ions from the storagetrap is typically 3 to 20 microseconds (for instance). The periodT_(axial)(m_(n)/z_(n)) of ion axial oscillations for fragment ions ofmass to charge ratio m_(n)/z_(n) is of the order of a few microseconds,depending on the value of M_(N)/Z_(N) or m_(n)/z_(n), of course. Thephase of oscillations P(m_(n)/z_(n),M_(N)/Z_(N)) can therefore bedetermined using equation 2 below:

$\begin{matrix}{{P\left( {{m_{n}/z_{n}},{M_{N}/Z_{N}}} \right)} = {{2{\pi \cdot {fraction}}\left\{ \frac{T_{\det} - {T\left( {{m_{n}/z_{n}},{M_{N}/Z_{N}}} \right)}}{T_{axial}\left( {m_{n}/z_{n}} \right)} \right\}} + c}} & (2)\end{matrix}$where P is the phase, c is a constant and fraction{ . . . } is afunction that returns a fractional part of its argument.

According to the Marshall reference cited above, the detected phase,P_(det)(ω), can be deduced by detecting the adsorption and dispersionfrequency spectra, A(ω) and D(ω) respectively as set out in equation 3below:

$\begin{matrix}{{P_{\det}(\omega)} = {\arctan\left\{ \frac{D(\omega)}{A(\omega)} \right\}}} & (3)\end{matrix}$and using the relation between the axial frequency of motion of ions ωand m_(n)/z_(n) for the orbitrapω(m _(n) /z _(n))=√{square root over (k·(m _(n) /z _(n)))}  (4)where k is a constant derived from the orbitrap's electric field. Theperiod of ion oscillations T_(axial)(m_(n)/z_(n)) is linked to the axialfrequency ω as

$\begin{matrix}{{T_{axial}\left( {m_{n}/z_{n}} \right)} = \frac{2\pi}{\omega\left( {m_{n}/z_{n}} \right)}} & (5)\end{matrix}$Thus, for a given fragment ion mass to charge ratio m_(n)/z_(n), andusing constants derived from a preliminary system calibration, it ispossible to deduce M_(N)/Z_(N), the mass to charge ratio of theprecursor ion from which the fragment ion of mass to charge ratiom_(n)/z_(n) is derived from equations 1 to 4. In other words,P(m_(n)/z_(n), M_(Z)/Z_(N)) is deduced from the measured phase andm_(n)/z_(n) (using equations 3 and 4) and from these values it ispossible to deduce T(m_(n)/z_(n), M_(N)/Z_(N)) from equation 2. As aresult, it is possible to deduce M_(N)/Z_(N) from equation 1. Thus, themass to charge ratio M_(N)/Z_(N) of a precursor ion from which afragment ion is derived can be unequivocally ascertained because theaxial oscillation of the fragment ion is linked to the phase of theprecursor ion oscillation in the orbitrap. This statement does, however,assume that m_(n)/z_(n) of a given fragment ion can arise only from asingle mass to charge ratio M_(N)/Z_(N) of precursor ion, and not alsofrom, say, M_(A)/Z_(A) or other precursor mass to charge ratios.

The initial phase of oscillation of the precursor and fragment ions inthe orbitrap is dependant on T which can be deduced from, for example,the real and imaginary parts of the Fourier Transform of the fragmention's axial oscillation frequency. Alternatively, T can be measureddirectly using TOF spectra acquired by the electron multiplier 190. Themass to charge ratio m_(n)/z_(n) could then be deduced using anappropriate calibration curve for the orbitrap. In this manner, all-massMS/MS spectroscopy is achievable.

However, the situation can be more complicated if two (or more)precursor ion groups having different M/Z (say, M_(A)/Z_(A) andM_(N)/Z_(N) produce a plurality of fragment ion groups having the samem/z (say, m_(n)/z_(n)). In any case, if fragment ions of the same massto ratio m_(n)/z_(n), (but derived from different precursor ions withdifferent mass to charge ratios M_(A)/Z_(A), M_(B)/Z_(B) . . .M_(N)/Z_(N)) enter the orbitrap at different moments in time, theiraxial oscillation frequencies are the same and so they are not otherwisedistinguishable from each other. This is so because the ion's frequencyof axial oscillations are independent of ion energy and initial phase ofion oscillation (i.e. it is only dependent on mass-to-charge ratio).

This situation can be exemplified as follows. Consider two groups ofprecursor ions with mass to charge ratios (say, M_(A)/Z_(A) andM_(N)/Z_(N)) respectively are released from the ion storage atsubstantially the same time and where M_(A)/Z_(A) is lower thanM_(B)/Z_(B) (mass M_(A) is lighter than mass M_(B)). As normal, the ionwith the lower mass-to-charge ratio moves faster than the heavier,followingTOF(M/Z)∝√{square root over (M/Z)}  (5)As a result, ions of mass to charge ratio M_(A)/Z_(A) arrives at the SIDsurface earlier than ions of mass to charge ratio M_(B)/Z_(B). Here, theions of mass to charge ratio M_(A)/Z_(A) promptly fragment, so that afragment ion with mass to charge ratio m_(n)/z_(n) is produced (alongwith other ions, of course). The specific ion under consideration, thatis, the ion with mass to charge m_(n)/z_(n), starts moving towards theorbitrap's entrance. If, for example, m_(n)/z_(n)<M_(A)/Z_(A) (which isnot always the case, for instance when m_(n)<M_(A), but z_(n)<<Z_(A)),then fragment ion m_(n)/z_(n) overtakes any M_(A)/Z_(A) precursor ionswhich did not fragment at the SID. Thus, according to equation 5 above,fragment ions with a mass to charge ratio of m_(n)/z_(n) arrive at theorbitrap's entrance before the unfragmented precursor ions. The timedifference of arrival at the entrance is governed by equation 1. It ispossible that, while the group of ions of mass to charge ratioM_(A)/Z_(A) are still in transit between the SID and the orbitrap'sentrance, the ion group having a mass to charge ratio M_(B)/Z_(B) arriveat the SID. Here they too fragment, forming (amongst others) a secondgroup of ions with a mass to charge ratio of m_(n)/z_(n), which proceedto move towards the orbitrap's entrance. As before, fragment ions in thegroup having mass to charge of m_(n)/z_(n) are likely to “overtake” ionsin the group having a mass to charge ratio M_(B)/Z_(B) on their way tothe orbitrap (assuming m_(n)/z_(n)). The second group of fragment ionsm_(n)/z_(n) arrive at the orbitrap's entrance after the first group offragment ions of the same m_(n)/z_(n) but deriving from the precursorions of mass to charge ratio M_(A)/Z_(A). As a result, the group offragment ions (with mass to charge m_(n)/z_(n)) arriving at theorbitrap's entrance first, and derived from the precursor ions of massto charge ratio M_(A)/Z_(A) has a different phase to the later group offragment ions with the same mass to charge ratio m_(n)/z_(n) but derivedfrom the other precursor ions of mass to charge ratio M_(B)/Z_(B). (Inextreme, and very unlikely, cases the phases of the two fragment iongroups can cancel one another out, resulting in no signal beingdetected).

If the electric field in the orbitrap is ideal (that is, perfectlyhyperlogarithmic) then both groups give a single spectral reading forthe same m_(n)/z_(n), regardless of the identity of the precursor ionsfrom which they derive, since (as explained previously), in an idealhyperlogarithmic field, the axial frequency of motion which is detectedis dependent only on m_(n)/z_(n) which is the same for each group offragment ions) and is not affected by any relative phase or energydifference between the two such groups This is undesirable since it isthen difficult to attribute the detected fragment ions (with mass tocharge ratio m/z), to one or other of a plurality of different precursorions. Thus, this signal needs to be unscrambled.

This unscrambling can be achieved by initiating the ramping of thevoltage 150 at a time before ions enter the trap, and to terminate theramp at a time after all the ions of interest have entered the trap. Asa result, a first group of fragment ions, that enter the trap at aearlier time than a second group of fragment ions, experience more ofthe ramped voltage than the second group, even for the same m_(n)/z_(n).Thus, the first group of ions are “squeezed” closer to the centralelectrode than the second group. As a result, the amplitude ofoscillation is therefore greater for the second group than the firstgroup. The first and second groups of fragment ions thus have distinctlydifferent orbital radii about the central electrode.

However, because the axial oscillation frequency is used for massanalysis in the orbitrap, and the axial frequency is not dependent onion energy or radius (or linear velocity as the ions enter theorbitrap), the first and second fragment ion groups have the same axialfrequency. As a result, they are still not resolved from one another inconventional mass analysis using the ideal E-field. Thus, using acalibration curve to determine the mass to charge ratio M_(N)/Z_(N) ofthe precursor ions (from equation 2) may produce a wrong assignment of agiven fragment ion to a precursor ion.

An aspect of the present invention provides a way to assign the fragmentions to their correct precursor ions. This is achieved by assessingdifferences in amplitudes of movement and energies of the ions in theorbitrap. This can be done by shifting the frequency of oscillation ofone group relative to the other (although as noted above the frequencyof axial oscillations in the orbitrap is normally independent of theseparameters.) The “frequency shift” can be introduced by distorting theideal electric field in the orbitrap in an appropriate manner.Preferably, the distortion is localised, for example, by applying avoltage to a (normally grounded) electrode disposed between, or near,outer detection electrodes.

It is preferable to charge the electrode to an extent that it distortsthe electric field away from the hyper-logarithmic field so that theions remain trapped, the ions amplitude of movement decays at a ratewhich does not prohibit efficient detection and the ideal field isdistorted so that ions of different energies and/or a sufficientfrequency shift is introduced between the two (or more) groups offragment ions with the same m_(n)/z_(n).

In a preferred embodiment, for trapped ions having energies of a fewkeV, a voltage is applied to the deflection electrode 200 to providelocalised distortion 202 to the trap field. The voltage is typicallybetween 20 to 250 volts, but may be higher or lower, depending on theenergy of ions in the orbitrap. As a result, the detected axialfrequency of ions oscillating relatively close to the distortion (thatis, the group of fragment ions of m_(n)/z_(n) which entered the orbitraplater resulting from the precursor ions of mass to charge ratioM_(B)/Z_(B), these fragment ions having a larger orbit radius), isdifferent from the fragment ions with the same m_(n)/z_(n) oscillatingfurther away from the distortion (that is, the group of fragment ionswhich entered the orbitrap at an earlier time, and derived fromprecursor ions of mass to charge ratio M_(A)/Z_(A)).

With reference to FIG. 3, a schematic diagram of the orbital paths 122,124 of two ions in an orbitrap 130 are shown. Both the ions have thesame mass to ratio; in the example outlined above, the two ions in FIG.3 would be ions in the two groups of fragment ions each of mass tocharge ratio m_(n)/z_(n). but deriving from precursor ions of mass tocharge ratio M_(A)/Z_(A) and M_(B)/Z_(B) respectively. Again, followingthe example above, the ion having a larger orbital radius (oscillationamplitude) 124 derives from precursor ions of mass to charge ratioM_(B)/Z_(B), whereas the smaller orbit 122 is followed by the ionderiving from precursor ions of mass to charge M_(A)/Z_(A). Theiroscillation frequencies along the trap's longitudinal axis z are,however, the same when an ideal hyper-logarithmic field is applied tothe ions, as discussed previously.

From FIG. 3, it can be seen that, when a voltage is applied to thedeflection electrode 200, the electric field in its vicinity isdistorted (as indicated at 202). Of course, the distortion is mostintense close to the electrode and diminishes as the distance from theelectrode increases. It can thus be seen that ions in the higher orbitalpath 124 experience the distorted field to a greater extent than ions inthe lower orbital path 122. Hence, the axial oscillation frequency (andphase) of ions in the higher oscillation amplitude path is affected (andshifted) to a greater extent than oscillation frequencies of ions inlower oscillation amplitude orbital paths. Thus, the detected massspectrum peaks for ions of the same mass to charge ratio m_(n)/z_(n),but having different precursor ions of mass to charge ratios M_(A)/Z_(A)and M_(B)/Z_(B) respectively, are split into separated, resolvablepeaks. Further, the initial phase of ions associated with each peak areresolvable.

With reference to FIG. 4, a voltage applied to the electrode used forintroducing the electric field distortion in the electrostatic trap,with respect to time, is shown. The voltage has two distinct stages, alow voltage stage 310 and a high voltage stage 320. The step 330 at timeT_(step) between stage 1 and 2 is relatively rapid so that the electricfield perturbations are introduced almost instantaneously. The voltagescale 340 in FIG. 4 only shows arbitrary values. The likely timerequired for each stage is preferably of the order of a few hundredmilliseconds to a couple of thousand milliseconds for stage 1 and of theorder of a few tens to a hundred milliseconds for stage 2. Thetransition between stage 1 and 2 should preferably be in the region of10 microseconds, or so. The voltage applied to the electrode duringstage 1 is chosen such that the electric field in the orbitrap is notdistorted. Hence, if the electrode to which the distortion voltage is tobe applied is disposed close to a normally grounded orbitrap electrode,then the initial voltage in stage 1 should also be ground, assuming thedistortion electrode is on the same equi-potential as the detectionelectrode.

With reference to FIG. 5, the amplitude 375 of a group of ions in anorbit in the orbitrap (again, for consistency with the explanation sofar, these would be fragmentations of mass to charge ratio m_(n)/z_(n)is shown with respect to time. It can be seen that the amplitude decaysrelatively slowly when the ions are trapped by an ideal Electric field.However, the amplitude decays at a very much faster rate when the idealfield is distorted after T_(D).

Referring to FIG. 6, a graph 400 of a mass spectrum resolved duringstage 1 (that is, no field perturbation in the orbitrap) is shown. Twopeaks 410 and 420 are shown, each having different intensities anddifferent mass to charge ratios. With reference to the previous exampleand the labelling conventions defined there, these mass to charge ratiosare for fragment ions, having mass to charge ratios m_(a)/z_(a) andm_(b)/z_(b) respectively. FIG. 7 shows a representation of the spectrumshown in FIG. 6 where the phase of the two peaks in FIG. 6 is shownagainst mass to charge ratio. The point 510 corresponds with peak 410 inFIG. 6 and the point 520 corresponds to peak 420 in FIG. 6.

Since the spectra shown in FIGS. 6 and 7 are taken during the firstacquisition stage, it is not possible to deduce whether any of thepoints in these spectra genuinely represent a single bunch of fragmentions, or whether they in fact represent more than one bunch of fragmentions, having the same mass to charge ratio but being derived fromdifferent precursor ions of different mass to charge ratios M_(A)/Z_(A)and M_(B)/Z_(B) (which will not, in stage one, be resolvable since herethe electric field is hyperlogarithmic). Expressed using the annotationas defined herein, the single peak 410 of FIG. 6 may be at m_(a)/z_(a)as a result of fragments of that mass to charge ratio from a singleprecursor of mass to charge ratio M_(A)/Z_(A) only, or it may instead bean unresolved peak representing fragment ions, all of mass to chargeratio m_(a)/z_(a), but deriving from two or more precursor ions of massto charge ratio M_(A)/Z_(A); M_(B)/Z_(B); M_(C)/Z_(C) . . . M_(N)/Z_(N).

Referring to FIG. 8, a spectrum similar to that of FIG. 6 is shown.However, the spectrum 600 in FIG. 8 is taken during stage two, that is,when a voltage is applied to the electrode to distort the electric fieldin the electrostatic trap 130. The group of peaks 601 to 604 correspondswith the peak associated with 410 of the spectra taken during stage one.Likewise, the group of peaks made up of peaks 611 to 614 are associatedwith the peak 420 of the spectra taken during stage one. Thus, it can beseen that each of the peaks of the spectra taken in stage one (when theelectric field in the electrostatic trap was homogeneous) is in factrevealed to be the unresolved consequence of a single mass to chargeratio m_(a)/z_(a) in the case of peak 410, and m_(b)/z_(b) in the caseof peak 420), deriving in each case from not one but four precursor iongroups (M_(A)/Z_(A); M_(B)/Z_(B); M_(C)/Z_(C) and M_(D)/Z_(D) for peak410, for example, and M_(E)/Z_(E); M_(F)/Z_(F); M_(G)/Z_(G) andM_(H)/Z_(H) for peak 420, perhaps).

FIG. 9 corresponds with the spectrum shown in FIG. 8 but the phase ofeach of the peaks in FIG. 8 is shown. Points 710 to 714 and points 711to 714 correspond to peaks 610 to 614 and 611 to 614 respectively. Thus,FIGS. 8 and 9, when compared with FIGS. 6 and 7 respectively, show howthe non-homogeneous electrostatic field in the orbitrap can be used to“split” spectrum lines to reveal the different precursor ion mass tocharge ratios responsible for a single mass to charge ratiofragmentation.

Faster signal decay and the resulting lower resolving power is expecteddue to the trap's inhomogeneous electric field, as shown in FIG. 5. Thepresent method should allow the separation of fragmented or precursorions whose mass-to-charge ratio are within a few percent of one another.If individual spectral peaks cannot be resolved then the correspondingfragment or precursor ion associated with the peaks can be flagged asunidentifiable.

It is preferable to acquire the data in two stages, as shown in FIG. 4.In stage one, the electrostatic field is maintained at an ideal state(or as close to this ideal as possible) so that the highest possibleresolving power and mass accuracy are obtained from the spectrometer.During stage one, the masses are measured to a high accuracy and anypossible isobaric interferences are also measured.

The system then switches to the second stage in which the electric fieldis perturbed by applying a voltage to an electrode close to one of theorbitrap electrodes. This perturbation causes spectral peaks to splitand thus facilitates fragment assignment. Preferably, the second stageis much shorter than the first stage. Both stage one and two arepreferably performed within a single spectrum acquisition.

The embodiments set out above are described with reference toelectrostatic trap mass spectroscopy. However, the methods may beapplicable to other forms of ion mass spectroscopy.

Variations of the apparatus and methods described above may also beenvisaged by a person skilled in the art. For instance, it may bepreferable to provide a dedicated electric field distortion electrode.This can be disposed on or off the orbitrap's equatorial axis. Theelectrode for distorting the electric field can be disposed at variouslocations in the orbitrap, some examples of which are shown in FIGS. 10to 13.

Referring to FIG. 10, the distorting electrode 500 is arranged as anannular ring electrode at either end of the central electrode 140. Withreference to FIG. 11, the distortion electrode 500 is disposed as aradial ring about the centre of the outer electrode 160. With referenceto FIG. 12, the outer electrode 160 is split into four parts comprisingtwo inner and two outer electrodes. During stage one of a spectralacquisition, all of the outer electrode components can be arranged tooperate at the same voltage to produce the ideal electric field.However, during stage two, a different voltage is applied to the twooutermost electrodes 510 to distort the ideal field. The electric fielddistorting electrode 510 should be arranged so that axial oscillationsof ions in the ideal field are generally within the inner edge of thedistortion electrode. Of course, the distortion electrode may also beapplied to the inner electrodes as well. Referring to FIG. 13, thedistorting electrode 520 is disposed on the central electrode. In thisexample, the distorting electrode is shown at a central position, but itcould also be arranged in any convenient location on the centralelectrode.

Other methods of distorting the electrostatic field will be apparent toskilled persons, other than the electrostatic distortion describedabove. For instance, resonant excitation of the ions by applying an RFvoltage to the electrode would be used to provide a dependence offrequency on the ion's parameters.

Also, the foregoing description refers to TOF ion separation. However,the present invention is not limited to only this method and other formsof ion separation, such as ejection from a linear trap for instance, maybe equally appropriate. For example, another embodiment of the presentinvention may include sequential ejection of precursor ions (which mighthave monotonously increasing or decreasing mass to charge ratios)towards the dissociation site. Thus, the TOF₁ term in equation 1 aboveis replaced with a scan dependent function. In practice, such a scancould be provided in different constructions of analytical linear traps,such as those described in U.S. Pat. No. 5,420,425 or WO00/73750.

1. A method of mass spectrometry using an ion trap, the methodcomprising: a) generating a plurality of precursor ions from a sample,each ion having a mass to charge ratio selected from a first range ofmass to charge ratios M₁/Z₁, M₂/Z₂, M₃/Z₃ . . . M_(N)/Z_(N); b) causingat least some of the plurality of precursor ions to dissociate, so as togenerate a plurality of fragment ions, each of which has a mass tocharge ratio selected from a second range of mass to charge ratiosm₁/z₁, m₂/z₂, m₃,z₃ . . . m_(n)/z_(n); c) directing the fragment ionsinto an ion trap, the ion trap including means for generating anelectromagnetic field which allows trapping of ions in at least onedirection thereof, the ions entering the trap in groups at a time whichdepends upon the mass to charge ratio of the precursor ions; d)determining the mass to charge ratio of ions in at least one of thegroups of ions, based upon a parameter of motion of the ions in that orthose groups in the said electromagnetic field in the trap; and e)distorting the electromagnetic field in the trap so as to permitseparate detection of fragment ions within the trap which have the samemass to charge ratio, but which are derived from different precursorions.
 2. The method according to claim 1, wherein: the step ofgenerating a plurality of fragment ions comprises generating, from afirst group of precursor ions having a first mass to charge ratio M₁/Z₁a first group of fragment ions having a mass to charge ratio m₁/z₁, andgenerating, from a second group of precursor ions having a second massto charge ratio M₂/Z₂, a second group of fragment ions also having amass to charge ratio m₁/z₁; and wherein the step of directing theplurality of fragment ions into the ion trap comprises directing thefirst and second groups of fragment ions, having the same mass to chargeratio m₁/z₁, into the ion trap, the groups arriving at the ion trap atdifference times because M₁/Z₁≠M₂/Z₂.
 3. The method of claim 2, whereinthe step of generating an electromagnetic field comprises generating anaxial ion trapping field in which ions oscillate in an axial directionof a potential well; and wherein the said parameter of motion employedto determine the mass to charge ratio of the ions is angular frequency,ω, the said angular frequency ω being dependent only upon the mass tocharge ratio of ions within it, so that fragment ions within the iontrap having a mass to charge ratio m₁/z₁ oscillate at the same frequencyω, regardless of the parameters of the precursor ion from which theyderive, prior to the said step of distorting the electromagnetic field.4. The method of claim 3, wherein the step (e) of distorting theelectromagnetic field comprises introducing a field component whichcauses the motion of the ions in the said potential well to becomedependent upon at least one further parameter so that fragment ionshaving the same mass to charge ratio m₁/z₁, but deriving from differentprecursor ions become distinguishable as a consequence of the dependenceof each separate group of fragment ions upon the said at least onefurther parameter.
 5. The method of claim 4, wherein the at least onefurther parameter includes a parameter selected from the list comprisingamplitude of motion in at least one direction of the trap; frequency ofmotion; the phase of a group in the trap; and the energy of ions in agroup in the trap.
 6. The method of claim 1, wherein the ion trap is anelectrostatic trap, and wherein the step of generating anelectromagnetic field therein comprises generating a substantiallyhyperlogarithmic field.
 7. The method of claim 1, wherein the step (e)of distorting the electric field comprises applying an additional localdistortion to the electric field, such that ions approaching the localdistortions distant from the local distortion within the trap experiencea substantially undistorted field.
 8. The method of claim 7, wherein theelectrostatic trap further comprises a distortion electrode, the methodfurther comprising applying a voltage to the distortion electrode so asto cause the said distortion of the electromagnetic field.
 9. The methodof claim 8 further comprising applying the distortion voltage to thedistortion electrode after a predetermined time has elapsed followingthe injection of ions into the ion trap.
 10. The method of claim 1,wherein the mass spectrum is obtained in two stages: in a first stage,the trap electromagnetic field is undistorted, and in a second stage,the electromagnetic field is distorted so that fragment ions having thesame mass to charge ratio, m₁/z₁, but which are derived from precursorions with different mass to charge ratios M₁/Z₁, M₂/M₂ can bedistinguished from one another.
 11. The method of claim 10, wherein thesecond phase commences after a predetermined period.
 12. The method ofclaim 1, wherein the step (b) of causing at least some of the precursorions to dissociate includes a technique selected from the listcomprising surface induced dissociation (SID), collision induceddissociation (CID), and photon induced dissociation (PID).
 13. Themethod of claim 12, wherein the step (b) of causing at least someprecursor ions to dissociate is through SID, the method furthercomprising applying a retarding voltage to a collision surface.
 14. Themethod of claim 13, wherein there is provided a collision surface, thecollision surface being arranged in a reflection geometry.
 15. A massspectrometer comprising: an ion source, arranged to supply a pluralityof sample ions to be analysed; means for directing the sample ionstowards a dissociation location, the sample ions arriving at the saiddissociation location as a plurality of groups of precursor ions inaccordance with their mass to charge ratios selected from the rangeM₁/Z₁, M₂/Z₂, M₃/Z₃ . . . M_(N)/Z_(N); an ion trap having a trapentrance, the ion trap being arranged to receive groups of fragment ionsgenerated by dissociation of the precursor ions at the dissociationlocation, each group of fragment ions having a mass to charge ratioselected from the range m₁/z₁, m₂/z₂, m₃/z₃ . . . m_(n)/z_(n), the iontrap further comprising trap electrodes configured to generate atrapping field within the ion trap, so that unfragmented precursor ionsand/or fragment ions entering the trap are trapped in at least one axialdirection thereof by the said trapping field and have a parameter ofmovement related solely to the mass to charge ratio of the ion;detection means to permit determination of the mass to charge ratio ofan ion group based upon the said parameter of movement; and at least oneelectric field distorting electrode arranged to provide a distortion ofthe trapping field so as to permit the detection means to detectseparate groups of fragment ions in the ion trap which have the samemass to charge ratio, m₁/z₁, but which have derived from precursor ionshaving at least two different mass to charge ratios M₁/Z₁, M₂/Z₂.